Field
Embodiments of the present invention generally relate to imaging systems and, more particularly, to ultrasonic imaging systems and methods of using the same.
Description of the Related Art
Ultrasound images are typically produced by generating an ultrasound wave traveling in a known direction and observing the echoes created when the sound wave bounces off of boundaries between regions of differing density in an object or passes through such regions when transmitted through the object. For any given direction, image pixels may be generated by plotting a dot whose brightness is proportional to an echo's amplitude at a coordinate whose location is a function of the time after a short pulse is sent in the direction in question. Alternatively, in the case of transmission, the image pixels may be generated by plotting the brightness of each dot according to the amplitude of the signal at the points where it is received after passing through the object.
The echoes from regions of different density, and from contrast agents such, for example, as microbubbles, are comprised of ultrasound signals at the transmitted frequency (the “fundamental frequency”) as well as signals at various multiples of the transmitted frequency (“harmonics”). Apart from the fundamental frequency, the strongest harmonic signal is generally at the second harmonic or twice the fundamental frequency.
In biomedical imaging applications, which are just one example of the myriad applications of ultrasound imaging, ultrasonic beams are subject to random scattering and distortion as they travel through regions of soft tissue, especially where there are acoustic interfaces as between muscle and fat. Collectively referred to as tissue aberrations, the scattering and distortion tend to degrade the clarity of an ultrasound image. In biomedical ultrasound, the second harmonic signal has been used intensively; in this case, noise and speckle are reduced by a one-way trip from the source tissue (compared with the round trip of the fundamental frequency from the probe to the object and back), resolution is enhanced due to the shorter wavelength, and reverberations/side-lobe artifacts are decreased. Because of these considerations, the harmonic signal is used to construct the final image, while the fundamental signal is disregarded.
The inventors herein have observed that echoes at more than one frequency (e.g., at the second harmonic and one or more other frequencies such as the fundamental frequency and/or another harmonic frequency) can be combined and/or differenced selectively, i.e., in a way that produces images having greater diagnostic clarity and/or utility than images produced utilizing echoes at a single frequency and/or by blindly applying subtraction on a systematic, pixel-by-pixel basis.